Why isn’t it “Esta puerta tine segura”?
I don’t understand your question, Seguro here means “lock” This door has a lock. 
If you want to use segura it would be “Esta puerta es segura” , This door is safe.
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Could you say, ‘Esta puerta tiene cerradura??’
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Yes you can. I live in Spain and that’s how I would say it.
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